This invention relates in general to heat exchangers; and, in particular, to an improved design noncontact heat exchanger as, for example, a heat recovery steam generator for a combined cycle power plant.
Heat recovery steam generators (HRSG's) are noncontact heat exchangers comprising a gas carrying duct having heat exchanger tubes mounted within the duct. As is shown in U.S. Pat. No. 4,345,438 issued Aug. 24, 1982 to Labbe and Dutremble and assigned to the assignee of the present invention, the heat recovery steam generator is an essential part of a combined cycle plant linking the gas turbine plant and the steam turbine plant. Hot waste gases from the exhaust end of a gas turbine are channeled into the heat recovery steam generator past the fluid carrying tubes to convert feedwater into steam. Because of their utilization of gas turbine exhaust gas heat, combined cycle plants are among the most efficient power plants in the world.
Despite the foregoing claim to high efficiency, manufacturers of power generation equipment continue to search for improvements which will provide even greater efficiency. The present invention provides increased efficiency through an improved sidewall baffle design.
The inventors of the present invention have discovered that gas passing along the sidewalls of the HRSG and not channeled through the heat exchanger tube bank can result in an HRSG efficiency loss of up to two percent for a realistic gap of merely one inch between the sidewall and the adjacent tube fin tip. Because of the foregoing discovery, the inventors set out to provide a means for decreasing gas flow in the spaces between the tube bank and either sidewall of the HRSG. One attempt at solving this problem suggested the use of stationary baffles fixed to each sidewall upstream and downstream from the tube bank. The stationary baffles extended inwardly from the sidewalls to overlap the respective flow path along the sidewall thus diverting flow into the tube bank. This solution was found to be unsatisfactory because of the thermal growth of the tube bank which would cause clearances between the tube bank and the sidewall baffles to change allowing the resumption of undesirable flow along the HRSG sidewalls. Other designs known to the inventors include the installation of half tubes on the flat sidewall and the use of contoured sidewalls. In both cases, less effective bypass prevention results from the less resistive flow blockage, coupled with thermal differential expansion and manufacturing tolerance effects. Therefore the inventors discovered that it would be desirable to design sidewall baffles as part of the tube bundle.